The technical problem to solve is that the tubes that act as condenser on their inside face and as evaporators on their outside face, such as the horizontal tubes of a shell and tubes heat exchanger, reach low heat transfer coefficients per square meter of tube surface and Celsius degree of temperature differential between the vapor that flows inside the tube and the liquid fluid evaporating on the outside surface of the tube.
In the specific case of shell and tubes exchangers for water desalination as used in MultiEffect Distillators MED, the thermal transfer coefficient of the horizontal tubes of these devices is below 3500 watts per square meter and per Celsius degree of temperature difference between the two faces of the tubes.
This limitation of the transmitted energy through the walls of current evaporator-condenser tubes restricts the number of condensation-evaporation cycles that may be realized with a source of energy at a given temperature, for example the steam from a cogeneration plant and a heat sink of lower temperature such as seawater, requires greater surfaces of exchange tubes inside the distillation devices and limits the total energy that may be managed by each distillation device.